Improved the handling of skipping tests for unsupported actions

Catching the UnsupportedDriverActionException everywhere to skip the tests
is totally ugly. Using the dedicated PHPUnit extension point is simpler,
and will support all tests easily.
Split the MouseOver event into several tests

Selenium1 and Selenium2 drivers are both marking this test as incomplete
and duplicating part of it for the events they support, so that these
events can be tested without being skipped because of other unsupported
events. Because of this, it does not make sense to have them in the same
test.
Added tests for all keyboards modifiers
Added a hook to wait before doing the assertion for PhantomJS
Merge pull request #466 from stof/improve_testsuite

Improve the testsuite